Emissive flat-panel display devices are widely used in conjunction with computing devices and in particular with portable devices. These displays are often used in portable devices or in public areas with significant ambient illumination. In these locations, the contrast of the display is of great concern.
In particular, OLED display devices suffer from problems with contrast. It is known to use a circular light polarizer affixed to the surface of the display so that light incident on the display is absorbed by the polarizer, while light emitted by the display is not. This is problematic in that the circular light polarizer is exposed to the environment and is subject to scratching, peeling, moisture, dents, and the like, which reduces its effectiveness and acceptability.
In an attempt to address the problem, WO0210845 A2 entitled “High Durability Circular Polarizer for use with Emissive Displays” published Feb. 7, 2002 describes a high durability circular light polarizer including an unprotected K-type polarizer and a quarter-wavelength retarder and designed for use with an emissive display module such as an organic light emitting diode or a plasma display device. Such devices are expensive and remain subject to environment stress which can degrade their performance. Moreover, placing a circular light polarizer on the surface of the display device inhibits the further integration of other elements such as lenslet arrays and touch screen components over the display.
An additional protective cover may be provided over the circular light polarizer. The LS633 digital camera, sold by Eastman Kodak Company, e.g., employed a polycarbonate cover located over the circular light polarizer to provide environmental protection to the OLED display and circular light polarizer. While such an approach is effective in protecting the display and circular light polarizer, it adds weight, size, and cost to the display. Moreover, modeling and tests done by applicant demonstrate that such a design severely reduces the contrast of the display.
There is a need therefore for an improved OLED display that improves the robustness of the display while maintaining the display contrast.